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Steel pistons for more efficient diesel engines

With a new cooling concept, an automotive industry supplier is expanding the range of applications for steel pistons in diesel engines. The Stuttgart-based company is testing a liquid alloy comprising sodium and potassium, which dissipates heat from the particularly thermally loaded piston head. This innovative steel piston concept is designed to enable even further increases in the specific power of car diesel engines. In the case of long-running engines for industrial applications and commercial vehicles, the component and engine oil load is significantly reduced while the expected service life and oil change intervals are prolonged.

With a new cooling concept, an automotive industry supplier is expanding the range of applications for steel pistons in diesel engines. The Stuttgart-based company is testing a liquid alloy comprising sodium and potassium, which dissipates heat from the particularly thermally loaded piston head. This innovative steel piston concept is designed to enable even further increases in the specific power of car diesel engines. In the case of long-running engines for industrial applications and commercial vehicles, the component and engine oil load is significantly reduced while the expected service life and oil change intervals are prolonged.The trend among automotive manufacturers is towards small but nevertheless powerful engines. With fewer cylinders and smaller cubic capacities, the friction, engine weight and moving mass are reduced. All in all, this downsizing produces more economical engines, especially in the partial load range. However, the greater thermal load on the components, especially on the pistons, limits this development. Their lower strength means that aluminium-based pistons are only suitable to a limited extent for the intended high ignition pressures. Although the steel piston concepts available today on the market with conventional oil cooling have a greater strength potential, the significantly poorer thermal conductivity of steel in this type of piston can lead to locally occurring temperature peaks. The piston head becomes particularly hot on the edge of the combustion bowl. This can cause cracks to occur during operation, which can cause the piston to fail.

Conventional pistons are sprayed with engine oil from below for cooling purposes. If they are designed for very high loads they also have an annular cooling channel within them through which engine oil flows via an injection port and at least one drain opening. Since this cavity is only partially filled, the engine oil is shaken up and down with the piston movement. This shaker effect ensures excellent heat transfer from the highly thermally loaded bowl edge to the oil-cooled cooling channel. It is essential that the oil in the cooling channel does not overheat. Otherwise, insulating coking residues will already begin to form on the channel walls even after short running times. The piston then overheats and fails prematurely, for example as a result of cracks forming on the edge of the bowl.

Liquid metal alloy instead of oil in the cooling channel

"We were therefore faced with the problem of removing the heat from the piston at temperatures that are higher than the engine oil can tolerate," says project manager Sascha Boczek in explaining the initial situation. "That's why we looked for a medium that still remains stable at temperatures even above 500 °C, and does not form oil carbon or other decomposition products in the oil."

The developers found a solution with a liquid metal alloy: "We're using the alkali metals sodium and potassium. In a suitable mixing ratio, they form an eutectic that is already liquid below room temperature. It has a high thermal conductivity and is even lighter than oil."

The special properties of this liquid metal alloy enable the heat flowing into the piston from above as a result of the combustion process to be distributed more uniformly and over a larger area. As a sealed cavity for transferring heat, the cooling channel therefore also has four blind holes along the wall of the piston. One of the holes is continuous and is used for filling purposes before this opening is permanently closed.

As a starting point for their experiments, the engineers modified a newly developed, serial-produced steel piston with a cooling channel (TopWeld) for the eutectic. They enlarged the annular cooling channel into which the eutectic is fed and reduced the wall thickness in the direction of the combustion chamber bowl. This improves the heat dissipation from the thermally highly loaded areas on the edge and bottom of the bowl, ensuring an overall more uniform temperature distribution in the piston. The spray nozzle that supplies the cooling channel with motor oil in conventional steel pistons now merely sprays into the inside of the piston for cooling purposes.

Measuring temperatures during operation

In order to measure the temperature of the pistons in the running engine, the engineers are using the Templug method. "Templug", which is derived from the words "temperature" and "plug", refers to a measuring pin made from a defined metal alloy that is used for measuring temperature. This pin is set to a calibrated hardness and is inserted at the measuring points in the piston. During the test programme, the operating temperature in the engine changes the micro-structure of the metal alloy and reduces its hardness. The scientists can derive the temperature once the experiment has ended based on the residual hardness.